1. Technical Field
This invention relates to an antilock braking device for precluding the locking of the wheels of a vehicle on sudden application of the brake and a road surface friction sensor and a road surface friction coefficient detector which can be used as components of the antilock braking device.
2. Technical Background
The conventional antilock braking device for cars or other vehicles generally employs a system such that the braking action is automatically controlled according to the chassis speed and wheel speed in such a manner that the slip ratio will fall within a definite range (see, for example, Japanese Patent Publication No. 30585/1984 and Japanese laid-open Patent Application KOKAI No. 61354/1985). The relationship between road surface friction coefficient and slip ratio is variable according to the texture of the road surface and, for this reason, the above system does not always provide the maximum braking force, depending on the condition of the road surface, and; in such cases, does not insure the minimum braking distance. Furthermore, because the chassis speed value used is an approximate value estimated from the wheel speed, the precision of slip ratio control is not sufficiently high. In order to ascertain the exact chassis speed, one has to rely on complicated devices such as a ground relative speed sensor (for example, Japanese laid-open Patent Application No. 64861/1988) or chassis deceleration sensor (for example, Japanese laid-open Patent Application No. 170157/1988).
In the conventional antilock braking device described in Japanese laid-open Patent Application No. 25169/1988, the road surface friction torque acting on the wheel (tire torque) is calculated from the wheel angular acceleration and brake fluid pressure values and the beginning of a fall in tire torque during the elevation of brake fluid pressure is utilized as one of the criteria for ascertaining the condition immediately preceding a wheel lock. However, since the tire torque is indirectly calculated from the wheel angular acceleration and brake fluid pressure, the above system does not take care of indefinite constants such as the moment of inertia of the wheels, the braking efficiency of the brake and so on, thus presenting problems in terms of the accuracy of data. There also is the problem that since the distance from the wheel to the road surface varies according to the deceleration of the chassis, depending on the pneumatic pressure of the tires and the weight of the chassis, the road surface friction force and the tire torque are not necessarily maintained in a fixed ratio.
It is an object of this invention to provide an antilock braking device free from the above-mentioned disadvantages of the conventional device.
It is another object to provide a road surface frictional force sensor and a road surface friction coefficient detector which can be used as components of an antilock braking device.